It has long been known that stirring can markedly improve the quality of glass, especially when done when the glass is in a relatively viscous state just before it is cast. With the recent emphasis on reduction of weight and wall thicknesses of many mass produced glass products such as, for example, television envelopes, non-returnable beverage bottles, improvement in the quality of the glass is becoming increasingly important. Striae, cords and other imperfections constitute localized weaknesses in the glass products, rendering them unmerchantable. Reduction of striae and other imperfections permits the achievement of lighter weight glass products because it enables a reduction in wall thickness without loss of strength relative to similar products in which account must be taken of considerable localized weakening due to the presence of cords and striae.
Thus, even for products which, heretofore, have been made of glass of relatively poor quality, there is a growing demand for glass of relatively high quality, of a quality even approaching that ordinarily required in the manufacture of optical ware.
Although the practice of the present invention is expected to have relatively wide application, its immediate utilization on a practical commercial scale is thought to have its greatest potential in connection with the standard type of industrial equipment on which substantially all of the mass produced glassware in the United States is made. The standard equipment includes a melting furnace where the initial batch ingredients of the glass are melted and heated to a relatively high temperature to form a glass of relatively low viscosity, a forehearth where the glass discharged from the melting furnace is cooled to make it more viscous, and a discharge bowl, or spout which receives the glass from the forehearth and discharges it through an orifice in the form of gobs. The apparatus of the present invention is especially suitable for substitution in place of the final section of the standard forehearth for stirring the glass just before it is delivered to the spout.
The standard forehearth comprises, in the order of flow of the glass from the melting furnace to the spout, first, a cooling section, and then a conditioning, or equalizing section. Both heating fires and cooling wind are provided in the cooling section, and heating fire is provided in the conditioning section but basically the design is intended to accomplish substantially all of the cooling in the cooling section, and to render the temperature of the entire body of flowing glass uniform throughout its thickness and width in the conditioning section. Heretofore, some stirring has been done at the entrance of the conditioning section by placing rotating paddles or turbines in the molten stream of glass at that point. Such stirring as heretofore carried out has been effective to improve the uniformity of temperature of the molten stream of glass and to reduce thermal gradients in it, but has failed in significantly reducing the amount of striae and cords in the glass and for physically homogenizing it.